Due to fine grain (usually less than 100 nm) and high interface density, metal nanocrystalline materials show unique mechanical performances and physicochemical performances, developing a new way for improving the comprehensive mechanical performances and service behaviors of conventional materials.
Most failure of materials occurs on the surface of the materials. For example, fatigue, corrosion and wear to materials are extremely sensitive to the surface structure and performances of the materials. Therefore, the surface structure and performances of materials have direct influence on its comprehensive performances. As a new method of surface modification, the metal surface nanocrystallization process utilizes the excellent performance of nano metal materials to prepare a superficial layer of nanocrystalline structure on a surface of traditional engineering metal materials, in order to improve the surface structure and performances of the metal materials, particularly, the fatigue resistance, corrosion resistance and friction and wear resistance, and to improve the comprehensive mechanical performances of the materials and therefore prolong the service life of the materials. Thus, nanocrystallization of metal material surfaces is of remarkable significance to the prolonging of the service life of the metal parts.
There are two major ways for nanocrystallization of a metal material surface. The first nanocrystallization way is surface coating or deposition, that is, heterogeneous materials are introduced in to form a nanostructured coating on a surface of the materials. The surface coating or deposition methods include Physical Vapor Deposition (PVB), Chemical Vapor Deposition (CVD), sputtering, electroplating, ejecting, etc. Due to the disadvantage of weak bonding between the coating and the substrate and between the coating particles, such way is likely to result in surface peeling or falling. Therefore, the application of such way in the industrialization is restricted. The second nanocrystallization way is mechanical surface grinding, that is, severe shaping transformation is mechanically caused on a metal surface, thus to refine the surface structure for the purpose of surface nanocrystallization. The mechanical surface grinding includes high-energy shot peening, ultrasonic shot peening, rolling, etc. There is no obvious interface between a nanostructured surface prepared according to this nanocrystallization way and a substrate structure, and no peeling or separation occurs; therefore, the performance in use of the materials is improved significantly. However, this nanocrystallization way has some limitations, for example, it is difficult to perform surface nanocrystallization on parts with curved surfaces, and disadvantages such as low productivity restrict the industrialization.
In an Invention Patent CN1924030A, entitled “Method for Nanocrystallizing Metal Surface by Using Ultrasonic High-energy Surface Machining”, a punch on an ultrasonic transducer is allowed to directly contact with a surface to be processed of a metal part to transmit the ultrasonic vibration energy to the metal part, and severe local plastic deformation at a high strain rate is performed on the metal surface, so that the grains on the metal superficial layer are refined to nanometer. The biggest advantage of that invention is that a layer of nanocrystalline with the same chemical components as the metal part substrate may be formed on the metal surface and the surface finish of the metal part may be significantly improved, without using any pellets or particles as intermediate energy carriers. The disadvantages of that invention is that, due to the limitations of the shape and size of the punch, the processed metal material surface is uneven with lumps and bumps and has high roughness, which may influence its surface property to some extent.